Many communications systems utilize cables having a plurality of twisted pairs of insulated conductors.
A communications cable utilizing twisted pair technology must meet stringent requirements with regard to data speed and electrical characteristics, such as a reduced cross-talk and a good electrical stability. When twisted pairs are closely bundled such as in a communications cable, disturbance of the signal transmitted by a twisted pair may occur due to electromagnetic interference between two different twisted pairs. Such phenomenon of signal disturbance, usually referred to as “cross-talk”, is highly undesirable and should be at least minimized if not eliminated altogether.
So, in the art of communications cables, the term NEXT (Near End Cross-Talk) indicates a transfer of energy from one pair to another measured between near ends (i.e. the disturbance caused on a receiving pair by a transmitting pair at the same end), the term FEXT (Far End Cross-Talk) indicates a transfer of energy from one pair to another measured between far ends (i.e. the disturbance caused to a receiving pair by a transmitting pair at the opposite end of the cable), while the term “power-sum cross-talk” indicates the overall transfer of energy towards one pair from all the other pairs.
Cross-talk especially presents a problem in high frequency applications because cross-talk increases logarithmically as the frequency of the transmission increases. At high frequency, furthermore, NEXT is the most relevant cross-talk phenomenon.
In an attempt to reduce the cross-talk phenomenon it was suggested in the art, as reported in U.S. Pat. No. 5,789,711, to use very complex lay techniques of the twisted pairs. In conventional cables, each twisted pair of a cable has a specified distance between twists along the longitudinal direction, that distance being referred to as lay length. When adjacent twisted pairs have the same lay length and/or twist direction, they tend to lie within a cable more closely spaced than when they have different lay lengths and/or twist direction. Twist direction may also be varied.
The use of such lay techniques to control the cross-talk phenomenon, however, has several disadvantages such as complexity, cost and susceptibility of the twisted conductors to electrical instability during use.
As an alternative remedy to reduce the cross-talk phenomenon, it was also proposed in the art, as reported in U.S. Pat. No. 5,789,711, to use shielded pairs of twisted conductors.
However, although being less prone to the cross-talk phenomenon, shielded cables are difficult and time consuming to install and terminate. Shielded conductors, in fact, are generally terminated using special tools, devices and techniques adapted for the job.
Shielding of twisted pairs is costly and complex to process and also susceptible to geometric instability during processing and use.
In order to reduce the cross-talk phenomenon, it was also proposed to use spacing means to space apart the twisted pairs of insulated conductors such as disclosed in U.S. Pat. No. 5,969,295. This reference describes a cable obtained by extruding a jacket around twisted pairs of insulated conductors reciprocally spaced by a cross-shaped spacer. In one embodiment, the jacket becomes integrally bonded to the radially outer tips of the spacer walls, thus defining a plurality of sector-shaped cavities each housing a respective pair of insulated conductors.
The cable disclosed by U.S. Pat. No. 5,969,295, however, is difficult to manufacture, possesses inhomogeneous mechanical properties and the construction thereof does not allow to prevent possible relative movements, albeit small, between pairs housed in adjacent sector-shaped cavities. In particular, on the one hand, the presence of the spacer may increase the stiffness of the cable, thus preventing an easy bending of the same, which bending is instead desirable for an easy installation of the cable. On the other hand, relatively low radial strains may instead damage the wall portions of the cross-shaped spacer, with an ensuing collapse of the cavities which could trigger the very cross-talk phenomenon which should be avoided.
Additionally, in order to hold the cable components together, the cross-shaped spacer and the cavities thereby formed in the cable of U.S. Pat. No. 5,969,295 are subjected to a helical torsion along the cable length, which requires an additional manufacturing step.
In another attempt to reduce the cross-talk phenomenon, EP-A-0 828 259 teaches to embed twisted pairs of insulated conductors within a flexible plastic material so as to stabilize the reciprocal position of the pairs.
However, a cable of this kind while showing, on the one hand, a satisfactory control of the cross-talk phenomenon coupled to a good mechanical resistance, is affected, on the other hand, by electrical and handling problems. A first problem which may occur is the difficulty of stripping the flexible material from the twisted pairs of insulated conductors without causing damage to the structure of the cable. A second problem is related to the possible permanent deformations which may occur whenever the bending radius of the cable is lower than a certain value. Such deformations may cause a variation in the impedance of the conductors, with consequent attenuation of the transmitted signal. Said variation of impedance is related to the “return loss” parameter, i.e. the ratio between the amount of power supplied to a conductor and the amount of power which is reflected along said conductor: the higher the parameter, the lower the attenuation. A third problem is related to the rigidity of this kind of cable which may render troublesome the handling and the installation of the same.